Online Biology Dictionary - M

m (1) meter; (2) minim.

m- (1) milli- (one-thousandth); (2) meta-.

μ- Micro- (one-millionth).

M (1) methionine; (2) molar (mol/L); (3) one thousand.

mAb Monoclonal antibody.

macroevolution The production during the course of evolution of new forms of life treated as distinct species.

macrofossil (also megafossil) A fossil large enough to be inspected with the naked eye. Compare: microfossil

macromolecule A large, complex molecule such as DNA, RNA or proteins.

macrophage A type of leukocyte produced by the division of monocytes, that protect the body by engulfing bacteria and debris. PICTURE OF A MACROPHAGE FORMING PROCESSES TO PHAGOCYTOSE TWO SMALLER PARTICLES

macropodid /mac-KRAW-puh-duhd/ A member of the mammalian family Macropodidae (/mac-kruh-PAWD-duh-dee/), which includes the kangaroos, wallabees, and wallaroos. ABOUT THE DIET OF KANGAROOS

magma /MAG-muh/ Molten silicate materials beneath the Earth's crust. Igneous rock is formed from cooling magma.

major histocompatibility complex (MHC) /hist-uh-cuhm-PAT-uh-bill-uh-tee/ A region composed of many separate genes that control the immune cells. In humans it is found on chromosome 6. The MHC is part of the major immunogene complex.

major immunogene complex (MIC) /im-MYOON-uh-jeen/ A region composed of loci encoding histocompatibility antigens, lymphocyte surface antigens, proteins of the complement system, and immune response gene products. It includes the major histocompatibility complex (MHC).

malignant /muh-LIG-nuhnt/ Cancerous. A malignant tumor can invade nearby tissue and spread elsewhere in the body

Malpighian tubules /mal-PIG-ee-uhn/ insect excretory organs that function in osmoregulation and cleanse the blood of nitrogenous wastes. They empty into the digestive tract.

Man Mannose.

ManN Mannosamine.

mandible /MAN-duh-buhl/ The bone of the lower jaw.

mannosamine (ManN) A hexosamine derived from mannose. MOLECULAR STRUCTURE

mannose (Man) /MAN-ose/ An aldohexose sugar. MOLECULAR STRUCTURE

marine /muh-REEN/ Of, pertaining to, or living in the sea.

marrow /MAIR-oh/ Flexible tissue present in the hollows of bones; new blood cells are produced in the marrow.

marsupial /mar-SOUP-ee-uhl/ A member of the major division Marsupialia of Class Mammalia. A marsupium is present in the females of many, but not all marsupials.

marsupium /mar-SOUP-ee-uhm/ (1) A fold of skin or pouch, containing the mammary glands; found in many, but not all animals classified as marsupials (MORE INFORMATION); (2) a pouch present in certain non-mammals, such as fish, in which eggs are carried externally.

mass spectrometer /speck-TRAWM-uh-ter/ An instrument that determines the chemical compound's composition on the basis of the mass-to-charge ratio of charged particles (used in DNA sequencing).

mastology /mast-TAWL-uh-jee/ The scientific study of the breasts.

maxilla /MACK-sil-uh/ A jawbone, especially of the upper jaw.

maxillary /MACK-suh-lair-ee/ Pertaining to the jawbone, especially of the upper jaw.

Mb (1) megabase; (2) myoglobin.

megabase (Mb) /MEG-uh-base/ A nucleotide sequence 1,000,000 bases in length.

megadont /MEG-uh-dawnt/ Having large teeth.

megafossil /MEG-uh-FAW-suhl/See: macrofossil.

megalocephaly /MEG-uh-low-CEF-uh-lee/Having an abnormally large skull.

megaspores /MEG-uh-spore/In the ovule of a flower, the four haploid cells produced by meiosis. Normally, only one goes on to produce a female gametophyte; the others degenerate.

meiosis /my-OH-suhs/ The process of two consecutive cell divisions that produces haploid sex cells and spores from diploid progenitor cells. Meiosis results in four daughter cells, each with a haploid set of unreplicated chromosomes. DETAILED DESCRIPTION OF MEIOSIS

meiospore /MY-oh-spor/ A spore produced by meiosis, which is therefore haploid and gives rise by division to haploid daughter cells, or to a haploid individual such as the gametophyte of a seed plant.

melanin (pronunciation) A group of tyrosine-derived compounds found in eukaryotes. Variation in melanin genes has its most evident effect in color variation. Eumelanin, which is brownish black, is the most common form. Also common is pheomelanin, which is reddish brown.

melanosomes /mel-LAN-uh-soams/Melanin-containing organelles derived compounds found in eukaryotes present in eukaryotic cells.

membrane /MEM-brane/ (1) A thin, flexible layer of tissue covering a bodily structure, or separating one structure from another; (2) the limiting surface of cellular cytoplasm, or of a cellular organelle; (3) a thin, flexible partition or covering of any type.

membrane potential Difference in charge between the cytoplasm and the fluid outside the cell; due to differences in the distribution of ions within and outside the cell.

menarche /mehn-AR-kee/ Onset of menses.

meninges /meh-NIN-jeez/ The membranes enclosing the spinal cord and brain.

meningitis /men-in-JYE-tuhs/ Inflammation of the meninges.

menses /MEHN-seez/ The monthly bloody discharge occurring in women after puberty.

meristem (often meristems) /MAIR-uh-stem/ Plant cells that remain embryonic, and that are therefore mitotically active, allowing plants to go on growing indefinitely as long as they live ("indeterminate growth").

meristematic /MAIR-uh-stuhm-MAT-ik/Of or relating to meristem.

meristic variation /mer-IST-ik/ Discrete variation that can be counted, as number of bristles, fingers, or body segments.

merogenesis /MAIR-uh-JEN-uh-suhs/ Reproduction by segmentation.

merogony (also schizogony) /mair-AWG-uh-nee/ A mode of asexual reproduction seen in certain protozoan parasites. In merogony, the feeding-stage form (trophozoite) of the parasite increases in size while its nucleus and other organelles divide repeatedly without cytokinesis. Then this enlarged, reproducing form (schizont) ruptures, releasing merozoites. This mode of reproduction is characteristic of apicomplexan parasites.

meropia /mair-OH-pee-uh/ Partial blindness.

merozoite /MAIR-oh-ZOH-ite/ In the life cycle of certain protozoan parasites, a stage produced by merogony.

mesenteries /MEZ-ehn-ter-eez/ Organ-suspending membranes found in the body cavities of vertebrates.

mesoderm /MEZ-uh-durm/ An early embryo's middle cell layer; develops into the muscles, skeleton, kidneys, spleen, gonads, and much of the circulatory system.

mesoglea (also mesogloea) /MEZ-oh-GLEE-uh/ The jelly separating the body walls of cnidarians and sponges.

mesophyll /MEZ-oh-fill/ The mid-layer of a leaf between the upper and lower epidermises; portion of a leaf where photosynthesis occurs.

Mesozoic Era /MEZ-uh-ZOE-ik/ The second of three geologic eras of the Phanerozoic Eon. It lasted from 251.0-65.5 mya.

messenger RNA (mRNA) RNA that serves as a template for protein synthesis. See also: genetic code.

met Methionine.

meta /MET-uh/ Chemistry: in meta-substitution, the substituents occupy positions 1 and 3. SEE EXPLANATORY DIAGRAM.

metabolism /muh-TAB-uhl-iz-uhm/ The chemical processes, taken as a whole, occurring within an organism.

metabolic pathway /met-uh-BAWL-ick/ A series of chemical reactions catalyzed by enzymes, occurring within an organism. The product of each successive reaction is the substrate of the next.

metamorphosis The transformation of a larva into the adult form.

metastasis Spread of cancer cells to new regions of the body.

methionine (M or met) An essential amino acid. CODONS | MOLECULAR STRUCTURE

methyl /METH-uhl/ The radical CH₃.

MHC Major histocompatibility complex.

MIC Major immunogene complex.

mice Small rodents belonging to the genus Mus. They are major vectors of disease. DISEASES CARRIED BY MICE

microfilaments Strands of actin present in almost all eukaryotic cells. Microfilaments are present in undulipodia and cilia, and make up part of the cytoskeleton. They act alone or with myosin to bring about contraction of the cell.

microfossil A fossil too small to be inspected without the aid of a microscope (less than about 4mm in diameter). Examples are the tests of microorganisms and pollen grains. Compare: macrofossil.

microinjection Injection of a DNA solution into a cell by means of a microcapillary pipet.

microorganisms Organisms too small to be seen with the naked eye.

microtubules /MIKE-roh-TOOB-yoolz/ Hollow strands of tubulin present in all eukaryotic cells where they are present in undulipodia and cilia, and make up part of the cytoskeleton. MORE ABOUT MICROTUBULES

minim /MIN-uhm/ One-sixteenth of a fluidram.

Miocene /MY-oh-seen/ An epoch of the Tertiary Period of the Cenozoic Era. It lasted from 23.03 to 5.33 mya. GEOLOGIC TIME SCALE

miscible /MISS-uh-buhl/ Capable of being mixed.

Cutaway view of a mitochondrion

mitochondria (sing mitochondrion) Organelles that are the site of respiration in eukaryotic cells.

mitochondrial DNA (mtDNA) The DNA found in mitochondria, the organelles that generate energy for the cell.

mitosis /my-TOE-suhs/ The process of eukaryotic cell division that produces daughter cells that are genetically identical to each other and to the parent cell. No change in chromosome number occurs. DETAILED DESCRIPTION OF MITOSIS

mitotic /my-TAWT-ik/ Of or relating to mitosis.

ml Milliliter.

μl Microliter (10^-6 liter).

mm Millimeter.

μm Micrometer (10^-6 meter).

mM Millimolar (mmol/L).

mmol Millimole.

model organism An organism studied as representative, either with respect to the course of some disease that also occurs in humans, or with respect to the general category of organisms to which the organism belongs.

mol Mole.

molar One of the grinding teeth at the rear of the jaw.

molarity /mole-AIR-uh-tee/ The number of moles of solute in a liter of solution.

mole The grams containing Avogadro's number of molecules of a substance; a mole of a substance has the same weight in grams as its molecular weight measured in daltons.

molecular biology The study of the formation, structure, and function of macromolecules found in living organisms, particularly nucleic acids and proteins.

molecular farming The development of transgenic animals to produce human proteins.

molecular genetics The study, at the molecular level, of the function and structure of genes.

Chambered Nautilus Nautilus pompilius. (click to enlarge)

mollusks (or molluscs) /MAWL-uhsks/ Animals in the phylum Mollusca, a category of organisms that seems to have no single defining characteristic applying to all of its members. The group is very diverse and includes animals such as gastropods, bivalves, octopuses, squids, cuttlefishes, and the chambered nautilus (at right). The mollusks have marine, freshwater, and terrestrial representatives.

molt /MOALT/ To cast off the outer covering. Birds molt old feathers once or twice a year. Retiles molt old skin, and arthropods cast off the entire cuticle. Mammals also molt hair, but the term shed is usually used in this case.

monandry /MAW-nan-dree/ The condition of having a single anthers.

monoclonal antibody (Mab) /MAW-no-KLONE-uhl/ An antibody obtained from a single clone of plasma cells.

monocot /MAWN-uh-kot/ Short for monocotyledon

monocotyledon /mawn-uh-kaut-uhl-EE-duhn/ An angiosperm with one cotyledon. COMPARISON OF MONOCOT AND DICOT SPROUTS

monocytes /MAW-nuh-sights/ A type of leukocyte that functions in the immune system by migrating to tissues and then diferentiating into macrophages and dendritic cells.

monoecious /muh-NEE-shuhs/ Having both carpellate and staminate flowers on the same individual.

monogenic disorder A disorder caused by mutation of a single gene.

monosaccharides (also monosaccharoses) /MON-oh-SACK-uh-rides/ Soluble simple sugars that cannot be broken down into simpler sugars of lower molecular weight; they are the simplest carbohydrates, acting alone or as monomers in disaccharides; the molecular formula of a monosaccharide is a multiple of CH₂O. Monosaccharides are unaffected by enzymes and are absorbed directly by the body without change.

monosomy Having one copy of a given chromosome instead of the two normally present in diploid cells. Compare trisomy.

monotreme /MON-uh-treem/ A member of the major division Monotremata (egg-laying mammals) of Class Mammalia, the other two being eutherian mammals and marsupials. Monotremata includes the platypus and the two echidnas.

morbid map A diagram showing the chromosomal location of genes associated with disease.

Morgan unit A unit (named for Thomas Hunt Morgan) expressing the relative distance between loci on a chromosome. One morgan (M) equals a crossover value of 100%, a decimorgan, a value of 10%, and one centimorgan, a value of 1%. In humans, a centimorgan is approximately equivalent to a megabase.

morphology (1) the structure and form of an organ or organism; (2) the scientific study of the structure and form of organisms.

mortification See: necrosis.

mouse See: mice.

mRNA See: messenger RNA.

mtDNA See: mitochondrial DNA.

multigene family See: gene family.

multiplexing A laboratory approach that performs multiple sets of reactions in parallel (simultaneously); greatly increasing speed and throughput.

murine Relating to the family, Muridae, to which rats and mice belong.

mutagen An agent that causes mutation.

mutagenicity The capacity of a chemical or physical agent to cause mutations.

mutant Affected by or having a mutation.

mutation (1) a change in the structure of a nucleotide sequence, often, but not necessarily, in a gene; (2) the altered sequence or gene resulting from such a change; (3) a change in the karyotype (chromosomal mutation) (4) an individual or type having such a change in its genetic makeup, and any descendants of that individual that in which it also occurs.

Mutualism: Clown Fish and Sea Anemone (click to enlarge)

mutualism A form of symbiosis in which both participants benefit. For example, the clown fish lives inside a sea anemone (see picture) and is protected by it. In return, it brings scraps to the anemone, and lures larger fish into the anemone's tentacles.

mV Millivolt.

mya Million years ago.

myalgia /my-AL-zhuh/ Muscle pain.

mycelium /MY-SEEL-ee-uhm/ A network of filaments (hyphae) that is a major component of the typical fungus.

Mycobacterium A genus of bacteria including the causative agents of leprosy and tuberculosis.

mycology The scientific study of fungi.

mycosis /my-KOE-suhs/ Any disease caused by a fungi.

myelin /MY-uh-lin/ A substance forming an outer layer, the "myelin sheath", normally found around a neuron. Myelin electrically insulates neurons and thus allows proper function of the nervous system.

myelocyte /MY-uh-luh-SIGHT/ The cells in marrow from which leukocytes are derived.

myelodysplasia /MY-uh-loh-dis-PLAZ-yuh/ Malformation of the spinal cord.

myelogenic /MY-uh-loh-GEN-ik/ (1) produced in the marrow; (2) producing marrow.

myoglobin /MY-uh-GLOH-bin/ A pigmented protein storing oxygen in muscle cells.

myosin /MY-uh-sin/ A protein that forms microfilaments in eukaryotic cells; interacts with actin to bring about cellular contraction; essential component of sarcomeres.